1. Field of the Invention
The invention concerns a device for producing plasma in a vacuum chamber with the help of alternating electromagnetic fields. According to the invention, a rod-shaped conductor is positioned inside of a pipe made of an isolating material and having an inner diameter larger than the diameter of the conductor. The isolating pipe is positioned inside of a vacuum chamber, whereby both ends of the isolating pipe are held in the walls of the vacuum chamber, and the outer surface of the pipe forms a seal between the chamber walls. Both ends of the conductor are then connected to a source for producing the alternating electromagnetic field.
2. Discussion of the Prior Art
With a known device for producing plasma (DE 195 03 205), it is possible to produce plasma for surface treatments and coating techniques within a limited operating range (i.e., with limited operating parameters, such as process area, gas pressure, and microwave output). The known device essentially consists of a cylindrical glass pipe installed in a vacuum chamber and a metallic-conducting pipe located within the glass pipe, whereby atmospheric pressure exists in the inner space of the glass pipe. Microwave output is initiated on both sides of the pipe through the walls of the vacuum process chamber by two power supplies and two metallic coaxial transmission lines, which consist of an inner conductor and an outer conductor. The absent outer conductor of the coaxial transmission line inside the vacuum process chamber is replaced by a plasma discharge, which is ignited and maintained by the microwave output under sufficient igniting conditions (gas pressure). The microwave output can be radiated from both the metallic coaxial transmission lines and the glass pipe in the vacuum process chamber. The plasma surrounds the cylindrically shaped glass pipe from the outside and forms, together with the inner conductor, a coaxial transmission line with a high attenuation coefficient. With a steady microwave output fed on both sides of the pipes, the gas pressure of the vacuum process chamber can be adjusted in such a way that the plasma apparently burns evenly along the device where the outside conductor of the coaxial transmission line is absent.
When the gas pressure in the vacuum process chamber is raised while a preset microwave output is applied, however, experience tells us that the uniformity of the plasma along the device is lost. The plasma at about half the distance between the power supply points of the device becomes optically weaker and can be completely extinguished beyond a certain pressure. When the pressure is raised further, the plasma line "tears apart" and the two existing plasma sections withdraw in the direction of the power supplies. Especially with long devices (e.g., 1 m or more) this effect leads to irregular plasmas and irregular vacuum processes resulting from it. The plasma sections display high luminosity at the ends near the walls and are weaker towards the middle. This can be attributed to an intrinsic behavior of coaxial transmission lines for which it obviously plays no role whether the outer conductor consists of metal or of electrically conductive plasma.
It is known that the attenuation .alpha..sub.c of the transverse electromagnetic waves is limited per unit of length of the coaxial transmission line by the electrical conductivity of the inner and outer conductors, and that this relationship can be formulated as follows: ##EQU1##
The parameters
a and b indicate the outer diameter of the inner conductor and inner diameter of the outer conductor of a coaxial transmission line; PA1 .delta..sub.S indicates the penetration depth (skin effect) of the microwaves in the conductive surfaces; PA1 .lambda..sub.0 is the free-space wavelength of the microwaves used, and PA1 .epsilon..sub.R indicates the relative dielectric constants of the dielectrics of the coaxial transmission line (=1 for air).
As one can infer from the formula, the attenuation of the microwaves does not in any way depend on the position along the coaxial transmission line. Because the dielectric filler of the coaxial transmission line consists of air and .epsilon..sub.R is constant over the length of the device, the magnitude of the attenuation depends only on the penetration depth of the microwaves in the conducting surfaces. When there is a constant attenuation per unit of length, this means that the net microwave output emitted per unit of length on the plasma decreases along the device toward the middle. Because the outside conductor consists of plasma, its conductivity cannot be determined exactly. Of course the conductivity of the outside conductor depends on the plasma density and this in turn is, to a limited extent, a function of the microwave power density in the discharge area. It is presumably several magnitudes higher than in the case of metallic surfaces (.about.50 .mu.m) and not constant over the length of the device.
Furthermore, it has been shown that the known device is rarely capable of maintaining a plasma discharge at pressures lower than 8.times.10.sup.-2 mbar. To raise the flexibility of the device it would be desirable to guarantee the operating conditions for a plasma discharge without the support of the magnetic field at even lower pressures.
The present invention is based on the technical problem of improving the disadvantages of the known device.